This invention relates to the treatment of solid carbonaceous material. It relates in particular to a process and installation for treating solid carbonaceous material.
According to a first aspect of the invention, there is provided a process for treating solid carbonaceous material, which process comprises heating the material to a temperature of about 1800xc2x0 C. or higher, by means of a non-transfer arc generated plasma flame, thereby causing components of, or present in, the carbonaceous material to be gasified and thus to be separated or removed from any residual solid material as a hot gas phase, with said any residual solid material being obtained as a product.
The non-transfer arc plasma flame is thus that produced by a non-transfer arc plasma generator or torch comprising an anode and a cathode between which an arc is generated while a gaseous medium passes between the anode and the cathode. The gaseous medium is heated by the arc, typically to a temperature in excess of 3000xc2x0 C., so that molecules of the gas split into atoms which become ionized and electrically conducting. The non-transfer arc generated plasma flame thus comprises the hot ionized gaseous medium, i.e. a plasma, and an elongate electric arc inside the plasma. The heating may be effected in a high temperature reaction zone in which the plasma generator or torch is located.
The solid carbonaceous material may be in particulate form, and may be present in the reaction zone in the form of a bed, such as a fixed or moving bed. The process may be a batch process. However, it is envisaged that the process will normally be a continuous process in which the bed of solid particulate material passes continuously through the reaction zone. More particularly, the bed of solid carbonaceous material may pass continuously through the reaction zone in a vertically downward direction, with fresh solid particulate carbonaceous material being added continuously to the top of the bed, and solid particulate product, when present, being withdrawn continuously from the bottom of the bed.
The process may include adding fresh solid particulate carbonaceous material to a preheating zone located above the reaction zone, and heating the carbonaceous material in the preheating zone by contacting it with the hot gas phase from the reaction zone. Thus, in the preheating zone, the feedstock may be heated from ambient or room temperature to a temperature of about 1800xc2x0 C., or higher.
The addition of the fresh solid particulate carbonaceous material to the top of the bed may be at such a rate that the bed moves through the reaction zone at a rate of from 10 mm/min to 90 mm/min, preferably from 40 mm/min to 70 mm/min.
The process may include cooling any residual solid material or solid product in a cooling zone below the reaction zone. The cooling may be effected by contacting the hot solid product with a treatment gas. The treatment gas may thus enter the cooling zone, cool down the hot solid product while it is heated, pass upwardly from the cooling zone into and through the reaction zone, and thereafter into and through the preheating zone. The hot gas phase then comprises spent or used treatment gas, gasified components from the solid carbonaceous material, and any gaseous products formed in the reaction zone.
The treatment gas may be reactive or non-reactive, depending on the solid carbonaceous material used.
The entire flow of treatment gas may thus be introduced through the cooling zone. However, in one embodiment of the invention, a portion of the treatment gas may be introduced directly into the reaction zone, eg directly with the plasma flame, as part of the plasma generating gas. In other words, at least a portion or component of the gaseous medium of the non-transfer arc generated plasma flame may be treatment gas. In this embodiment of the invention, about a third of the total treatment gas required in the reactor, may then enter as part of the plasma generating gas.
The treatment gas may be selected from: an inert gas such as argon, helium and neon; a relatively inert gas such as nitrogen; a more reactive gas such as oxygen; a gas which is liquid at ambient conditions, such as superheated steam, which can then typically be at a temperature in the range of 1000xc2x0 C. to 1800xc2x0 C.; a synthesis gas such as a hydrogen-carbon monoxide mixture; a halogen such as chlorine or fluorine; and a mixture of two or more of these gases.
It will be appreciated that, in the event that no solid produce is produced, treatment gas can still be used in the process, with the make-up and addition rate of the treatment gas thus constituting one of the process variables. The make-up and addition rate of the gaseous medium or plasma generating gas can also constitute one of the process variables.
The process is characterized thereby that differing feedstocks can be treated in the process, and the process variables or parameters can be selected to obtain different products. Thus, in one embodiment of the invention, only the solid carbonaceous material may be used as a feedstock to the preheating zone, ie the feedstock consists only of the solid carbonaceous material. The solid carbonaceous material may be selected from coke such as synthetic grade coke, pitch grade coke or petroleum grade coke; waste carbonaceous material such as waste anode material; anthracite; and coal. The process can then be operated to obtain purified carbonaceous material as the solid product, with the components which are gasified in the reaction zone being impurities or undesirable components present in the feedstock material. Instead, the process can be operated to obtain one or more desired gasified or gaseous components are present in the hot gas phase, with any residual solid material being of little or no value. The components present in the gas phase may then be recovered as products.
In another embodiment of the invention, a mixture of the solid carbonaceous material and a solid non-carbon material selected from a metal or metal-containing mineral, eg chromite; a non-metal compound such as an oxide, eg silica oxide, particularly an oxide capable of being converted to a carbide; and a ceramic, may be used as a feedstock to the preheating zone. The process can then be operated to obtain a valuable solid product other than coke.
In yet other embodiments of the invention, the make-up and/or addition rate of the treatment gas and/or the plasma generating gas can be varied or altered to obtain both valuable gaseous components and valuable solid products, in some cases. The treatment gas make-up will be dependent on the particular feedstock that is used, as well as the recovery system used to recover valuable components from the gas phase.
The feedstock may have a purity between 70% and 99.9% (by mass), and more typically between 80% and 99.9% (by mass), so that it then contains between 0.1% and 20% (by mass) impurities. When coke is used as feedstock, or as a component thereof, the impurities are typically present in the form of 1.0% to 1.6% bonded nitrogen, and at least 0.2% sulphur (by mass).
The particle size distribution of the feedstock is typically from 1 mm to 30 mm in diameter or cross-section, preferably from 3 mm to 15 mm.
As stated hereinbefore, the feedstock is heated to a temperature of about 1800xc2x0 C. or higher, ie the reaction temperature in the reaction zone is not less than about 1800xc2x0 C. At temperatures below about 1800xc2x0 C., the removal of impurities such as nitrogen and sulphur from carbonaceous material such as coke is excessively time consuming. The upper limit of the temperature to which the material is heated, is set by the formation of undesirable species in the gas phase. Typically, however, the maximum reaction temperature may be between 1800xc2x0 C. and 2600xc2x0 C., when the feedstock consists only of carbonaceous material such as coke. Preferably, the reaction temperature is then at least 2000xc2x0 C., eg between 2100xc2x0 C. and 2300xc2x0 C. Typically, the maximum reaction temperature may be up to 4000xc2x0 C., when the feedstock comprises a carbonaceous material in admixture with metal oxides, ceramics or halide containing compounds, as hereinbefore described.
Any hot solid product may be cooled down, in the cooling zone, from the temperature to which it has been heated in the reaction zone, eg 2100xc2x0 C. to 2300xc2x0 C., to 1000xc2x0 C. or less, eg to as low as 150xc2x0 C.
The pretreatment zone, the reaction zone, and the cooling zone may be thus provided in a single vessel or reactor, which may be a vertical shaft non-transfer arc plasma reactor.
The required residence time of the solid material in the reaction zone to obtain a desired conversion is a function of the reaction temperature, and may typically be between 10 minutes and 8 hours, preferably between 30 minutes and 60 minutes, at a reaction temperature of between 1800xc2x0 C. and 2500xc2x0 C. Since the reactor is filled with the solid material, the residence time of the material in the reaction zone is controlled by the withdrawal rate of solid product from the cooling zone.
Heat transfer in the reactor is determined by the actual velocity or mass flow rate per unit of cross flow area of the treatment gas through the reactor, the gas temperature and the temperature difference between the gas and the solid material.
The radial temperature profile in the reactor is thus largely determined by the gas mass flux, gas temperature and heat loss to the reactor surface at any vertical position of the reactor. The solid material or phase characteristics, such as voidage, particle size distribution, particle surface area, density, conductivity, temperature and mass flow will also influence this temperature profile. Any reactive combination between the gas and solid material will also affect the temperature profile.
Typically, the mass ratio of the feedstock to the treatment gas is between 1:0.1 to 1:8.
The power input to the plasma generator is typically between 200 kW and 6000 kW, preferably between 600 kW and 3000 kW.
The typical energy usage of the process is between 0.1 MWh/t and 25 MWh/t, eg between 0.7 MWh/t and 1.5 MWh/t, for carbonaceous material. The carbonaceous material may undergo a crystal structure change due to high temperature treatment and the thermal conductivity of the solid material can be increased from typical values of 5 W/mK to more than 20 W/mK, and typically to about 50 W/mK.
The temperature profile of the material in the reaction zone for a specific reactor, is controlled or determined by means of the power input to the plasma generator, the treatment gas composition, the percentage of the treatment gas that is introduced with the plasma flame, the overall treatment gas flow rate through the reaction zone, the characteristics of the solid material particles, the reactor insulation, the reactor diameter profile, the particle mass flow rate, and the reactor height.
If desired, in addition to cooling down any solid product with the treatment gas, further cooling thereof can take place, within the cooling zone and/or outside, eg adjacent, the cooling zone, eg by means of a water cooled extractor.
By means of the treatment gas, energy is thus recovered from the hot solid carbonaceous product. When the treatment gas comprises a relatively inert gas such as nitrogen, it will typically have a purity of at least 97% (by mass). The mass ratio of treatment gas to feedstock material within the reactor, and without any treatment gas recovery, is between 0.6 and 1.5. This indicates that with treatment gas recovery and recycling, the usage ratio of treatment gas to feedstock material may be between 0.01:1 to 0.6:1.
The solid product, when present, is, when the process is a continuous process, continuously extracted from the bottom of the reactor at a predetermined rate, eg at between 0.1 and 10.0 tonnes per hour. The product, which is typically in the form of pellets or particles, is naturally dependent on the feedstock used, the reaction temperature, and the treatment gas. Typically, the product may comprise purified coke, containing no graphite, when the feedstock consists only of impure coke and the reaction temperature is about 1800xc2x0 C.; purified coke, containing graphite, when the feedstock consists only of impure coke and the reaction temperature is about 2500xc2x0 C.; activated carbon when the feedstock is coke, anthracite or coal and steam is used as the treatment gas or as part of the gaseous medium; a metal, when the feedstock is a specific metal or mineral/carbon mixture at residence reaction times and temperature; a carbide ceramic, when the feedstock is a specific silicon oxide/carbon/metal mixture.
However, carbonaceous by-products can also be recovered together with the main product from the bottom of the reactor, when the feedstock does not consist only of carbonaceous material. Thus, when the main product recovered is a metal or a ceramic, then a typical carbonaceous by-product which is recovered is purified carbonaceous material.
The gas phase withdrawn from the reactor typically includes components such as CN, HCN, CCl4, CF4, C2F4, CO, H2 or CxHy (where x and y are integers), or combinations thereof.
The gas phase withdrawn from the reactor, is the effluent gas, can be recycled after cooling and cleaning thereof. This will reduce the make-up or pure treatment gas requirement, eg to approximately 0.1 kg fresh gas feed per kg of fresh feedstock into the reactor.
The slow moving bed within the reactor results in a minimal abrasion effect, and the feedstock particles generally do not decrease significantly in size. The amount of fines lost to the gas phase is typically less than 3% (by mass).
According to a second aspect of the invention, there is provided an installation for treating solid carbonaceous material, which comprises
a vertical shaft non-transfer arc plasma reactor comprising an upper preheating zone, an intermediate reaction zone in which at least one non-transfer arc plasma generator or reactor is located, and a lower cooling zone;
solids feed means for feeding a feedstock comprising solid carbonaceous material, into the preheating zone;
gas phase withdrawal means for withdrawing a gas phase from the preheating zone;
solid product withdrawal means for withdrawing a solid product from the cooling zone; and
treatment gas feed means for feeding a treatment gas into the cooling zone.
It will be appreciated that there will not usually be a clear demarcation, within the reactor, of the various zones which can, in use, shift within the reactor depending on the control parameters; however, during operation of the reactor, the three zones will be present within the reactor with there thus being an overlap between adjacent zones. Additionally, in use, the fixed or moving bed of feedstock material will thus fully occupy the cooling zone, the reaction zone and the preheating zone; however, if desired, a head space may be provided in the preheating zone to facilitate disengagement of the hot gas phase from the fresh feedstock material at the top of the bed.
In view of the high temperatures present in the reaction zone of the reactor in use, there may be provided, in at least the reaction zone of the reactor, a heat resistant refractory lining of carbon; graphite; ceramic; zirconium, eg zirconium oxide; a carbide; or a metal such as molybdenum or tungsten. The thickness of the lining may be from 1 mm to 1 m, depending on the reaction zone and the process design parameters. Additionally, the chemical and mechanical properties of the lining are selected to be compatible with the feedstock, the treatment gas, the gas phase, the plasma generating gas, etc.
The installation may include a hopper arrangement or the like for introducing the feedstock into the top of the reactor. The hopper arrangement can then form part of the reactor, ie may be located immediately above the preheating zone.
Other aspects of the installation and the reactor, such as the non-transfer arc plasma generator, may be as hereinbefore described with reference to the process of the first aspect of the invention.
The invention will now be described by way of the enclosed drawing and the subsequent non-limiting postulated examples. The reference numerals referred to in the examples are those indicated in the drawing.